1. Field of the Invention
The present invention relates to a hydraulic system having valves that are operated to control the flow of fluid to hydraulic actuators that move components on a machine, and more particularly to distributed control systems in which the valves are located adjacent the associated hydraulic actuator being controlled.
2. Description of the Related Art
A wide variety of machines are operated by hydraulic systems. For example, a backhoe is a common type of earth moving equipment that has a bucket rotatably attached to the end of an arm that in turn is pivotally coupled by a boom to a tractor. A hydraulic boom cylinder raises and lowers the boom with respect to the tractor and a hydraulic arm cylinder pivots the arm about the end of the boom. The bucket is rotated at the remote end of the arm by a hydraulic bucket cylinder.
Traditionally, the boom assembly was controlled by valves located near the cab of the tractor and mechanically connected to levers which the operator manipulated to independently move the boom, arm and bucket. A separate valve assembly was provided for each cylinder on the boom assembly. Operating one of the valve assemblies permitted pressurized hydraulic fluid to flow from a pump on the tractor to the associated cylinder and other fluid to return from that cylinder back to the tank on the tractor. A separate pair of hydraulic conduits ran from each valve assembly adjacent the operator cab along the boom assembly to the associated cylinder.
There has been a recent trend away from mechanically operated valves to electrohydraulic valves that are operated by electrical signals. Initially, all of the electrohydraulic valves were mounted on a single manifold block, such as the one described in U.S. Pat. No. 6,505,645, that was centrally located on the machine. Pairs of hydraulic conduits ran from that common manifold block to each hydraulic actuator on the machine. The use of electrohydraulic valves eventually evolved to the development of a distributed hydraulic system in which the valve assembly is collocated with the associated hydraulic actuator, such as a cylinder. With this type of system, the operator in the tractor cab manipulates joysticks or other input devices to generate electrical control signals for operating the valve assemblies. Because each valve assembly is adjacent the respective hydraulic actuator, the amount of plumbing on the machine is reduced. Now only a pair of conduits, a supply conduit and a tank return conduit, extends along the boom assembly to power the cylinders for the boom, arm and bucket on a backhoe, for example. Electrical cables run from a central electronic controller for the machine to the valves on the assemblies near the hydraulic actuators.
Other types of equipment also incorporate such distributed hydraulic systems.